King Argilac knew where the Ironborn would soon turn. You would think a king who wanted to keep his crown would be weary of a man with fire-breathing monsters, but King Argilac Durrandon wasn't called "the Arrogant" for nothing. Determined to arrest decline, he sent an envoy to Dragonstone to enlist Aegon Targaryen and his dragons against his enemies. In exchange, Argilac offered lands he didn't have and a wife Aegon didn't need. For as the Valyrians did, Aegon had wed his elder sister Visenya. Then as the Valyrians didn't, he took the younger Rhaenys as well. Two sisters, two wives. Perhaps that's why he was so keen to get off Dragonstone.

Aegon countered Argilac with courtesy. He sent his own envoy requesting that Princess Durrandon's hand be given instead to Aegon's closest friend and rumored half-brother Orys Baratheon. Argilac answered with a box and a note: "These are the only hands your bastard shall have of me!" Inside the box were the hands of Aegon's envoy. How long had Aegon been waiting for such a pretext? As his armies prepared to sail, ravens flew to every great lord of the Seven Kingdoms. All bore the same message: From this day forth there would be but one king in Westeros. Those who bent the knee to Aegon of House Targaryen would keep their lands and titles. Those who took up arms against him would be thrown down, humbled and destroyed. Poor old Argilac. He couldn't even match Aegon in arrogance.

Aegon landed at the mouth of the Blackwater River and raised a primitive Aegonfort in the disputed lands between Harren's and Argilac's kingdom, so that neither could decide whose problem he was. Adopting the customs of the Seven Kingdoms, Aegon unfurled his own banner with a red three-headed dragon breathing fire upon a black field. Visenya crowned him with a ruby-studded circlet of Valyrian steel while Rhaenys hailed him as the first high king the continent had seen since the Dawn Age. As his lords and the gathered locals cheered him, Aegon named his loyal friends to a Small Council led by Orys Baratheon, whom he called "my strong right hand". The title stuck, and a bastard became the first Hand of the King.

But for now Aegon's kingdom contained only a rocky island and a handful of fields. The other kings had large armies, ships, castles and thousands of years of rule. But Aegon had dragons. He had chosen for the words of his house "Fire and Blood". Before he was done, the rivers, fields, and skies would turn red.

Notes

The video mentions that it was traditional for Valyrians to incestuously marry brother to sister, but it was uncommon for them to marry more than one wife at the same time (though not unheard of). Visenya was Aegon I's older sister, while Rhaenys was his younger sister: it is believed that he married Visenya out of duty, but then married Rhaenys out of love - his relationship with Visenya was always somewhat less close. Under the inheritance law of the Andals in Westeros, a daughters rank behind sons in line of succession, even if a daughter is older than the firstborn son - which can be a source of succession conflicts. Apparently, the Valyrians tended to just sidestep the issue by having a firstborn daughter marry her firstborn brother, explaining why Aegon married Visenya out of "duty".

As Maester Yandel has pointed out, the name "Seven Kingdoms" for all of Westeros south of The Wall was always something of a misnomer: there simply happened to be seven kingdoms at the time when Aegon invaded. They used to be divided into many more in prior centuries, the Riverlands were a contested borderland that hadn't been independent for centuries, and Dorne was technically a "principality", not a kingdom. Moreover, Yandel specifically cites that no one ever counted the small Targaryen holdings on the islands in Blackwater Bay (such as Dragonstone) as an eighth "kingdom", even though they are considered part of Westeros (albeit not on the mainland).

This video briefly describes that the domains of the Storm Kings had been gradually diminishing. A previous Histories & Lore video on "The Stormlands" narrated by Brienne of Tarth explained some more detail from the books on this: the Storm Kings of House Durrandon expanded to conquer the Riverlands and hold it for three centuries, creating a super-sized kingdom that ruled almost half of Westeros south of the Neck. They also conquered strips of territory along their borders with the Reach and the Dornish Marches. Unfortunately their conquests over-expanded, and now had hostile land borders with all the other major kingdoms, not just two or three (somewhat similar to how in the real-life Middle Ages, the amorphous Holy Roman Empire expanded in all directions, and ruled vast swaths of territory, but ended up overextending itself). The local River lords also hated them and constantly revolted. Over time the Durrandons came under pressure from all directions, until ultimately the Iron Islands invaded and captured the Riverlands from them - during the time of Argilac's grandfather. Then in the time of Argilac's father, the ironborn conquered further east, down to the mouth of the Blackwater (the region that later became the Crownlands). Argilac himself was a strong ruler and did much to stave off further losses during his prime, but by the time of the Targaryen invasion he was an old man with no sons (only a daughter), and he couldn't hold off the ironborn forever. The "lands he didn't have" that he offered Aegon were those north of the Blackwater (the future Crownlands), which Harren Hoare still held, meaning Aegon would have to conquer them himself, and then remain as a buffer territory fighting off the ironborn so the Durrandons didn't have to.

As the video directly explains, the dragonlord families of Old Valyria didn't really use heraldry the way that noble Houses in Westeros did: Aegon I himself invented the official heraldry of House Targaryen when he invaded Westeros: a red dragon on black, with three heads (referring to the three Targaryen siblings, and the dragons they rode). This was a symbolic act which placated many new lords who switched to Aegon's side when the invasion began, a sign that he was embracing Westerosi culture. This also means that previous generations of Targaryens who ruled Dragonstone (such as Aenar Targaryen) never used this heraldry, as Aegon I hadn't invented it yet.

Aegon I Targaryen did create the first royal Small Council, and name Orys Baratheon as his first Hand of the King, but the council's structure wasn't totally formalized until the reign of his grandson, King Jaehaerys I Targaryen. Certain offices like Grand Maester or Master of Whisperers might not have existed until after Aegon I died, and it may not have been as formal of a free-standing "organization" until the time of Jaehaerys I.

The Histories & Lore animated videos have to guess when it comes to the color of the dragon Vhagar. Balerion was stated to have been black since the first novel, while The World of Ice and Fire revealed that Meraxes was silver/white. Vhagar remains probably the most prominent dragon whose color has never been mentioned - to the consternation of many fans. The co-authors of The World of Ice and Fire said that they get asked this frequently, but the simple answer was that even when they repeatedly asked George R.R. Martin, he said he just hadn't gotten around to deciding it, even after all these years.